Australia is currently having an election and understandably the media reports new political poll results daily. In a country of 22 million what percentage of the population would need to be sampled to get a statistically valid result?
Is it possible that using too large a sample could affect the results, or does statistical validity monotonically increase with sample size?
Answer
Sample size doesn’t much depend on the population size, which is counter-intuitive to many.
Most polling companies use 400 or 1000 people in their samples.
There is a reason for this:
A sample size of 400 will give you a confidence interval of +/-5% 19 times out of 20 (95%)
A sample size of 1000 will give you a confidence interval of +/-3% 19 times out of 20 (95%)
When you are measuring a proportion near 50% anyways.
This calculator isn’t bad:
Attribution
Source : Link , Question Author : brotchie , Answer Author : Neil McGuigan